It is a needless 12mm Bing carb on there. They used a more powerful 12mm on the 1D, and a weaker 14mm on a lot of the As, Bs, and Cs. I don't know why.
"A... comes with a 14mm diameter intake."
"B... comes with a 12mm diameter intake."
"C... comes with a 14mm diameter intake."
"D... paired with a 26mm diameter exhaust and a unique 12mm square bing that has no needle."
I have also heard that although the 12mm on the 1D is good, a 15mm Bing or Dellorto is better and the AMAL is the best, but a little more complicated.
I was asking because I wanted to hear from someone with personal experience. The wiki isn't always the best source. For example, it seems that most of the people who worked on the wiki and shop at treats don't even know how to put on a simple exhaust.
I got the Giannelli pipe, the one that so many complain it falls off constantly. I simply used a rubber mallet to to tap the pipe all the way on and it stays on so tight that I didn't even need a bracket or to use the screw that squeezes the exhaust port. I road around with just friction holding it on for a week and it didn't move. I finally made a bracket "just in case", but when I was putting it on, I noticed the pipe hadn't even slipped half a mm.